Heated substrate support station for sputtering systems

ABSTRACT

An improved substrate support for sputtering systems permits simultaneous heating and application or either direct current or radio frequency bias voltages to the substrates. The station includes a hollow support electrode having a flat surface for receiving the substrates. The electrode is attached to a vacuum chamber by a combined electrical insulating and vacuum sealing means. An inner dark space shield mounted within and insulated form the support electrode prevents glow discharge from the internal surfaces of the electrode and also provides a shielded recess for radiant heating means.

July 31, 1973 R IEH 3,749,662

HEATED SUBSTRATE SUPPORT STATION FOR SPUT'IER'ING SYSTEMS Filed April 17,- 1972 2 Sheets-Sheet 1 24 5 i Amer 25 2 643 Mar Z0 71 July 31,. 1973 BIEHL 3,749,662

HEATED SUBSTRATE SUPPORT STATION FOR SPUTTERING SYSTEMS Filed April 17, 1 972 2 Sheets-Sheet 2 United States Patent 3,749,662 HEATED SUBSTRATE SUPPORT STATION FOR SPU'ITERING SYSTEMS Richard E. Biehl, Pearl River, N.Y., assignor to Materials Research Corporation, Orangeburg, N.Y. Filed Apr. 17, 1972, Ser. No. 244,563 Int. Cl. C23c 15/00 US. Cl. 204-298 10 Claims ABSTRACT OF THE DISCLOSURE An improved substrate support for sputtering systems permits simultaneous heating and application or either direct current or radio frequency bias voltages to the substrates. The station includes a hollow support electrode having a flat surface for receiving the substrates. The electrode is attached to a vacuum chamber by a combined electrical insulating and vacuum sealing means. An inner dark space shield mounted within and insulated from the support electrode prevents glow discharge from the internal surfaces of the electrode and also provides a shielded recess for radiant heating means.

BACKGROUND OF THE INVENTION This invention relates to sputtering apparatus and more particularly to a substrate support station for use in such apparatus.

The process of depositing a thin film of one material upon an item of different material by gas ion bombardment of a target made of the first material is known as sputtering. This processis assuming increasing commercial importance in the production of homogeneous thin films of elements, alloys, or compounds on a variety of substrates.

Apparatus for sputtering typically includes a seal able chamber, a vacuum pumping system for evacuating the chamber, and a source of electrical energy. The electrical energy can be either high negative potential direct current (DC) or high potential radio fiequency (RF). DC power is sometimes used with conductive targets, while RF power is necessary when sputter depositing from nonconductors. The items to be coated, called substrates, are placed on a conductive substrate support station, and an item of the desired coating material, called a target, is mounted on a target support plate facing the substrate station in the chamber. The chamber is then sealed and evacuated, typically to a pressure of about 10'' torr (i.e., 10" mm. of mercury) to remove most of the surface contaminants on the target and substrate materials as well as atmospheric gases and water vapor. An inert gas, such as argon, is then bled in until the chamber reaches a pressure sufiicient to support a glow discharge, usually in the range of about to 35 microns (1 micron=- mm. of mercury).

The source of negative DC (3000 to 4000 volts) or RF energy is then connected to the target, the substrate station usually being maintained at or near ground potential. The target thus becomes the cathode and the substrate station the anode of a gas discharge system. The flow of electrons from cathode to anode ionizes the intervening gas, and the positive argon ions are accelerated by the electrical field toward the target. Bombardment of the target by these ions causes ejection of particles of target material which are deposited on the substrate.

In many cases it is desirable to provide either a relatively low negative DC bias voltage or a small amount of RF power to the substrate station, depending upon whether DC or RF power' is used, rather than to maintain the support at ground potential. This so-called DC or RF biasing operates as a film cleaning process during sputtering deposition from the target. In sputtering, because of the relatively high argon gas pressure involved, and because of outgassing from sputtering fixtures, substantial amounts of gas can be incorporated into deposited films. The biasing voltages help to eliminate this gas entrapment. As the film is deposited, it is lightly scrubbed by ions which remove the loosely held gas atoms without disturbing the adherent sputtered film. A denser and more adherent film results.

It is usually also desirable to preheat the substrate before starting the sputtering process. Conventionally, substrate heating is performed at a different station from sputter deposition, if biasing is going to be used. The reason for this is the difficulty in providing both a biasing voltage and heating current simultaneously to the same support station. This is a particular problem when using RF biasing.

In US. Pat. 3,369,989, issued on Feb. 20, 1968, to E. Kay et al., for example, a heated substrate support station is disclosed that has no provision for supplying a bias voltage. The Kay support includes a heavy-walled, hollow metal cylinder with a resistance wire heating element sandwiched between layers of electrical insula tion. The interior of the cylinder is sealed against the vacuum in the sputtering chamber, and cooling fluid is supplied through flexible metal hoses. The heavy-walled construction of Kay et al., required because the interior is maintained at atmospheric pressure, makes it diflicult to rapidly heat the substrate support surface and to maintain the surface at a constant temperature.

Kay et al. also disclose that it is conventional to surround the cathode of a sputtering system with a shield for suppressing glow discharge and to provide a hollow support to carry cooling fluid to the base of the cathode. The interior of Kays hollow support must be sealed from the chamber vacuum. The support and particularly the flat target, must be of relatively heavy construction to withstand the pressure diiferential between the chamber and the fluid-cooled interior.

It is an object of the present invention to provide a hollow substrate support station that can be simultaneously electrically heated and biased with either a DC or RF potential.

It is a further object of the invention to provide a hollow substrate support station in which the interior communicates with the pressure in the sputtering chamber.

It is .a further object of the invention to provide a substrate support station including a relatively thin support plate which can be rapidly heated and which has a thin edge to reduce heat loss by conduction from the plate.

SUMMARY OF THE INVENTION The hollow substrate support station of the present invention includes a portion with a flat outer surface for receiving the substrate. Insulating means attach the support to a conventional sputtering vacuum chamber so that the support may be maintained at a diiferent electric potential than the chamber. An inner dark space shield mounted within and insulated from the hollow support electrode prevents glow discharge within the electrode. The means for insulating the dark space shield from the support electrode preferably also provides a vacuum seal, thus permitting the inside of the electrode to communicate with the evacuated space of the sputtering chamber. Radiant heating means, preferably an electrical resistance coil, is located within the inner dark space shield to heat the flat surface of the support electrode. The insulating and vacuum sealing means are preferably far enough away from the fiat support surface to prevent differential heating of the viarous members that could lead to loss of sealing capability. In addition, cooling means can be provided in the vicinity of the vacuum seals to insure that no differential thermal expansion occurs.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of a DC sputtering system including the substrate support station of the present invention.

FIG. 2 is a schematic diagram of an RF sputtering system including the substrate support station of the present invention.

FIG. 3 is a detailed side view in cross-section of the preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The improved substrate support station of the present invention can best be understood by reference to the overall sputtering system diagrams of FIGS. 1 and 2. In these diagrams like components will be designated by the same number.

Referring first to the system of FIG. 1, the typical sputtering system includes a vacuum chamber having a cathode 12 and a substrate station 14 mounted in opposed facing relation. The sputtering chamber 10 may be in the form of a cylinder with transparent walls 16 of tempered glass enclosed by a top plate 18 and a bottom plate 20. A vacuum connection 22 leads to a vacuum pump (not shown) for evacuating the chamber to the desired vacuum of at least 10- torr. An inert gas such as argon may be bled in through inlet 24 controlled by a valve 26. Usually argon is fed in continuously with the pumping system in operation, the argon feed rate being adjusted to provide a chamber pressure of about 5-35 microns. The continuous gas flow serves to purge the chamber of contamination liberated during the sputtering process.

Sputtering discharge will take place when cathode 12 is connected to the negative terminal of DC supply 28, and substrate station 14 is maintained at or near ground potential.

As described in the introduction, station 14 may be DC biased by connecting it to a slightly negative voltage of about -50 volts, for example, as is illustrated schematically in FIG. 1. This negative DC potential attracts a portion of the gas ions in the chamber to lightly scrub the surface of the substrates of contamination during the sputtering process. To suppress glow discharge from the side and back surfaces of both cathode 12 and substrate station 14, these electrodes are surrounded respectively by dark space shields 30 and 32.

Referring to FIG. 2, the equivalent RF sputtering systern includes the same elements with the exception of the power supply. In place of the DC supply of FIG. 1, FIG. 2, shows an RF supply 34 for providing power at 13.56 mHz., a frequency officially assigned for industrial and scientific purposes. Because radio frequency energy of this frequency is transmitted through hollow wave guides instead of through solid conductors, it is a simple matter to provide a portion of the power supplied by RF source 34 through a T coupling 36 and waveguide 38 to substrate station 14, with the main power flow going to cathode 12 through waveguide 40. A typical distribution of power is 90-95% to cathode 12 and 5-10% to substrate station 14. The effect of RF biasing is similar to that achieved with DC biasing; the advantage of RF is that it can be used with nonconductive substrates, and conducting substrates which have non-conducting surface layers.

In the preferred embodiment shown in detail by FIG. 3, substrate support station 14 includes a hollow cylindrical support electrode 42 having an upper end 44 of enlarged diameter and a lower end 46 of reduced diameter. The upper end of the support terminates in a flat support surface 48. The support is positioned in radially spaced relation through a hole in bottom plate 20 of chamber 10 by means of an insulating ring 50 claimped in sealing relation between the lower end of support electrode 42 and plate 20. O-rings 52 and 54 provide a seal against loss of chamber vacuum between the wall of the chamber and the outside surface of the substrate support.

An inner dark space shield 56 that conforms substantially to the shape of the inner surface of substrate support electrode 42 is positioned concentrically within the substrate support body by means of a second insulating ring 58 clamped between the substrate support and the bottom of the shield. O-rings 60 and 62 provide a vacuum seal so that the interior of the substrate support may be maintained at chamber vacuum.

Inner dark space shield 56 contains a recess 64 at its top in which is placed a radiant heating element 66. Radiant heating element 66 is preferably of the metal sheathed electrical resistance type similar to the conventional heating element used in electric ranges. Alternatively, an uninsulated bare Nichrome wire element could be used. Leads 68 to heating element 66 pass through a sealing means 70 in the base of inner dark space shield 64 to a source of electrical current (not shown).

Cooling fluid such as water is supplied through conduits 72 and 74 for circulation around portions of the support electrode 42 and inner dark space shield 64 that are adjacent to insulating seal ring 58. In this way, the metal clamping structure on both sides of seal ring 58 can be maintained at approximately equal temperatures to avoid thermal stresses and possible loss of vacuum seal between O-rings 60 and 62 and the surface of ring 58. Electrical lead 76 connects inner dark space shield 64 to the bottom plate 20 of the vacuum chamber, so that both are maintained at the same electrical potential which is normal ground potential.

An important feature of the preferred embodiment of FIG. 3 is that flat support surface 48 is not sealed to the top of support electrode 42. In fact, surface 48 may be perforated if desired. In this Way the interior of substrate support station 14 sees the same pressure as the rest of the chamber, and support surface 48 can be made of relatively thin metal without danger of deformations from the fiat surface necessary to provide good electrical contact when the substrates are placed on it. By making support surface 48 of thin metal it can be rapidly heated by heating element 66. In order to reduce conductive heat loss from the edges of surface 48, it is possible to provide an annular region 78 of reduced thickness where surface 48 meets support electrode 42.

The connections to support electrode 42 from either a DC or RF biasing source are conventional and are not shown in FIG. 3. If desired, the support electrode 42 can be made the cathode of the sputtering system by applying sufiiciently negative DC voltage or sufiicient RF power in order to conduct so-called sputter etching of the substrates prior to depositing of a sputtered film from normal cathode 12.

From the foregoing description, it is apparent that the substrate support station of the present invention provides a compact, convenient station which permits maximum flexibility in the choice of sputtering techniques and substrate materials.

-I claim:

1. In sputtering apparatus of the type including a vacuum chamber, a target and a substrate mounted in opposed relation in the chamber, and a source of electrical energy for causing material to be removed from the target and deposited in a thin film on the substrate by sputtering action, an improved substrate support station comprising:

(a) a hollow substrate support electrode having a portron with a fiat outer surface for receiving the substrate;

(b) insulating means for mounting the support electrode to the chamber, whereby the support may be maintained at a different electric potential than the chamber;

(c) an inner dark space shield mounted within and insulated from the hollow support electrode to prevent glow discharge from the internal surfaces of said electrode; and

(d) radiant heating means within the inner dark space shield for heating said flat surface of the support electrode.

2. The support station of claim 1 wherein said insulating means comprises a vacuum seal between the chamber and the support electrode.

3. The support station of claim 2 further comprising cooling means for said support station in the vicinity of said insulating means to prevent diiferential thermal expansion at the vacuum seal.

4. The support station of claim 1 further comprising means for electrically connecting said inner space shield to the chamber, whereby the inner space shield is maintained at the same electrical potential as the chamber.

5. The support station of claim 1 wherein said portion of the support station having a flat outer surface comprises a flat circular plate supported at its circumference and having an annular region of reduced thickness adjacent the circumference to impede conductive heat transfer from the plate to the remainder of the support station.

6. The support station of claim 5 wherein said flat circular plate is perforated.

7. In sputtering apparatus of the type including a vacuum chamber, a target and a substrate mounted in opposed relation in the chamber, and a source of electrical energy for causing material to be removed from the target and deposited in a thin film on the substrate by sputtering action, an improved substrate support station comprising: (a) a substrate support having a hollow cylindrical body, the interior of which is exposed to chamber vacuum, with an upper end of enlarged diameter, terminating in a flat, relatively thin support surface, and a lower end of reduced diameter extending in radially spaced relation through a hole in the chamber;

(b) a first insulating ring clamped in sealing relation between the substrate support body and the chamher;

(0) an inner dark space shield having a hollow cylindrical body with an upper end of enlarged diameter and a lower end of reduced diameter positioned concentrically within the substrate support body, the outer surface of the dark space shield being spaced from the inner surface of the substrate support so as to suppress glow discharge inside the hollow substrate support; and

(d) a second insulating ring clamped in sealing relation between the dark space shield and substrate support body.

8. The support station of claim 7 further comprising:

(a) a radiant heating means positioned at the upper end of said inner dark space shield adjacent the under face of the flat support surface of said substrate support.

9. The support station of claim 8 further comprising cooling means for the region adjacent said second insulating ring.

10. The support station of claim 8 wherein said radiant heating means comprises a flat, metal-sheathed, electrical heating coil.

References Cited UNITED STATES PATENTS 3,485,739 12/1969 Toombs 204192 3,595,775 7/ 1971 Grantham et al. 204298 3,630,881 12/ 1971 Lester et a1. 204298 3,661,761 5/ 1972 Koenig 204298 JOHN H. MACK, Primary Examiner S. S. KANTER, Assistant Examiner US. Cl. X.R. 204--192 

